print errors on stderr, not stdin
Rewrite to use HTTP for communication

In previous revisions the communication between the browesr and bctl was
made with a custom protocol. That was nice but doesn’t work well for
other browsers. So I switched to a more generic protocol other HTTP.

Consequences :

 * read() and readline() are gone. They were buggy and probably
   impossible to do it "right" anyway (javascript can’t block), so…
 * The firefox part is now a standalone extension
 * Expect to see a PhantomJS backend soon